Vapor and Liquid Phase Profiles of Essential Oils from Abies, Picea and Pinus Species and Their Phytotoxic Interactions with Weed Growth in Pre- and Post-Emergence Conditions

The chemical content of essential oils (EO) obtained from the leaves of four Pinaceae (Abies alba, Picea abies, Pinus cembra and Pinus mugo) was investigated by SPME-GC-MS technique. The vapor phase was characterized by the monoterpenes with values higher than 95.0%. Among them, α-pinene (24.7–48.5%), limonene (17.2–33.1%) and β-myrcene (9.2–27.8%) were the most abundant. The monoterpenic fraction prevailed over the sesquiterpenic one (≥74.7%) in the EO liquid phase. Limonene was the major compound in A. alba (30.4%), P. abies (20.3%) and P. mugo (78.5%), while α-pinene in P. cembra (36.2%). Regarding the phytotoxic properties, EOs were studied at different doses (2–100 μL) and concentrations (2–20/100 μL/mL). All EOs were found to be significantly active (p-value ˂ 0.05) against the two recipient species in a dose-dependent way. In pre-emergence tests, germination of Lolium multiflorum and Sinapis alba was reduced by up to 62–66% and 65–82%, respectively, as well as their growth by up to 60–74% and 65–67%, due to the effects of compounds in both the vapor and liquid phases. In post-emergence conditions, at the highest concentration, the phytotoxicity of EOs caused heavy symptoms and, in the case of S. alba, A. alba EO completely destroyed (100%) the treated seedlings.


Introduction
Weeds are plants capable of causing qualitative and quantitative damage in terms of the yield and commercial value of agricultural products. These plants have characteristics that can increase the damage they cause, such as germination in adverse conditions, production of very long-lived and easily dispersible seeds, high competitiveness and rapid growth [1]. To have successful and sustainable crop production, it is necessary to develop alternative control strategies against weeds.
The chemical approach of using synthetic herbicides, often chosen for their speed and ease of use, is becoming increasingly problematic in terms of environmental pollution due to their low biodegradability and high persistence and percolation of the active compounds into soil and water [2]. Furthermore, due to the use of pesticides with similar mechanisms of action, resistance phenomena are also increasing [3]. Because of these issues, various formulations and active ingredients potentially capable of exerting an allelopathic action have been studied to be integrated with traditional weed control methods [4].
Allelochemicals must be released from the donor plant in a variety of ways, such as leaching from leaves and stems, volatilization from plant parts, and release by decomposition of organic residues or as root exudates. The phytotoxicity of these compounds are attributed to their ability to interfere with the biological mechanisms of the target plant at a biochemical (e.g., synthesis of DNA and protein alteration), physiological (e.g., respiration alteration) or structural (e.g., cuticle and cell membrane modification) level. The global effect of these mechanisms can be translated into reduced germination, a lower growth of shoots and roots, a lower absorption of nutrients and a decrease of the photosynthetic rate [5].
Essential oils (Eos) are among the products with allelopathic activity. When effective, their action causes rapid drying of the green parts of the plant by destroying the leaf cuticle and cell membranes [6]. EO-based herbicides have already been patented as natural and non-toxic products that can be used as a safe alternative for weed control in organic farming systems [7]. According to Verdeguer et al. [1], five commercial organic herbicides derived from EOs were available on the US market prior to 2020.
In this work, we focus on the phytotoxic potential of EOs obtained from four Pinaceae species, such as Abies alba Mill., Picea abies (L.) H.Karst., Pinus cembra L. and Pinus mugo Turra. To this end, Lolium multiflorum Lam. and Sinapis alba L., weeds of wheat in centralsouthern Italy and characterized by phenomena of resistance to conventional herbicides, including glyphosate [8,9], were used as receiver plants both in pre-and post-emergence tests. The volatile chemical profiles of the four EOs, both in vapor and liquid phase, were characterized by SPME-GC-MS and GC-MS techniques, respectively.

Vapor Phase Chemical Composition of EOs
SPME-GC-MS analysis was performed to delineate the vapor phase profile of the EOs. In total, 55 compounds were identified, with 15 in A. alba, 18 in P. abies, 21 in P. cembra and 16 in P. mugo (Table 1). α-Pinene was the main compound in A. alba (48.5%) and P. cembra (52.9%), together with β-myrcene in P. abies (24.7% and 24.8%, respectively) and the secondmost abundant component in P. mugo (28.0%), where limonene (33.1%) prevailed. Limonene was also detected at the same percentage (33.1%) in A. alba as well as in a slightly smaller amount in P. cembra (29.3%). β-myrcene was missing in A. alba, but significantly present in P. cembra (9.2%) and P. mugo (27.8%). The only ubiquitous sesquiterpene compound was β-caryophyllene which ranged from 0.1% to 0.4%. A series of compounds belonging to this chemical class were detected but with relative percentages lower than 0.1%. The exception was δ-cadinene which in P. mugo reached 0.3%.

Liquid Phase Chemical Composition of EOs
By gas chromatography-mass spectrometry (GC/MS) analysis, the liquid phase profile of EOs was described. A total of 69 compounds were identified, with 30 in A. alba, 45 in P. abies, 32 in P. cembra and 27 in P. mugo (Table 2). In all samples, the monoterpene fraction prevailed over the sesquiterpene fraction. Among the monoterpenes, limonene was the most abundant compound in A. alba (30.4%), P. abies (20.3%) and P. mugo (78.5%), while α-pinene in P. cembra (36.2%); however, significant differences in composition were found. For example, β-pinene (17.6%) and β-myrcene (11.3%) reached higher relative percentages in P. abies than in other EOs where they varied between 0.2% and 9.4%. Bornyl acetate, on the other hand, reached higher amounts in A. alba (7.3%) and P. abies (7.1%) than in P. mugo (1.7%) and P. cembra (1.5%). Among the sesquiterpenes, β-caryophyllene was the main compound in A. alba (9.6%), P. abies (3.7%) and P. mugo (2.0%) while δ-cadinene in P. cembra (2.4%).    1 The components are reported according to their elution order on apolar column; 2 Symbol for class compound, m-monoterpenes. s-sesquiterpenes. 3 Linear Retention indices measured on apolar column. 4 Linear Retention indices from literature. 5 Percentage values of A. alba EO components (%). 6 Percentage values of P. abies EO components. 7 Percentage mean values of P. cembra EO components (%). 8 Percentage mean values of P. mugo EO components. -: Not detected; tr: traces (mean value <0.1%). Table 3 shows the germination and growth values of L. multiflorum and S. alba whose seeds were treated (without direct contact) with four different doses of EOs. In general, both receiver species were significantly affected by the four EOs (p-value < 0.05). L. multiflorum was more susceptible to the action of EO from P. abies and A. alba, S. alba to that of P. cembra and P. mugo. P. abies EO was more effective than A. alba and P. cembra than P. mugo.  Specifically, at the highest dose (100 µL), P. abies reduced germination of L. multiflorum by 62%, followed by P. cembra (−46%), P. mugo and A. alba (−39% and −38%, respectively). P. abies EO was the only one to significantly inhibit it (−57%), even at a dose of 50 µL.
Concerning seedling growth, EOs from A. alba and P. abies, at 100 µL, similarly reduced shoot elongation of L. multiflorum (by about 60%), while P. cembra inhibited it by 42% and P. mugo by 28%. Slightly higher percentages were reached in the developmental arrest of S. alba, even if by different EOs (P. cembra > P. abies > P. mugo > A. alba). All EOs were significantly active against both receiver species even at 50 µL (up to −55% for L. multiflorum and up to −42% for S. alba) while their lowest doses (2 and 20 µL) against S. alba only. Table 4 shows the germination and growth values of L. multiflorum and S. alba whose seeds were treated (direct contact) with four different concentrations of EOs. As in the previous test, both receiver species were significantly affected by the four EOs (p-value < 0.05), albeit differently. Again, L. multiflorum was more sensitive to the activity of A. alba and P. abies, but in this case, A. alba had a stronger effect than P. abies. S. alba was mostly affected by P. abies and P. mugo, with some exceptions. In general, A. alba EO was more effective than P. abies and P. cembra than P. mugo.   In detail, A. alba was able to decrease the germination of L. multiflorum more than P. abies, P. cembra and P. mugo (up to −66%, −51%, −47% and −42%, respectively) while being active only at 20 and 10 µL/mL, unlike other EOs that were effective even at lower concentrations (2 and/or 5 µL/mL). In contrast, A. alba did not significantly inhibit S. alba germination, unlike P. abies, P. cembra and P. mugo, which reduced it by up to 65%, 43% and 55%, respectively.

Phytotoxicity of EOs in Liquid Phase
The EO impact data on the germination of the receiver species were corroborated by the relative indices. At the four used concentrations, A. alba decreased GI and CVG values of L. multiflorum similarly (−28% to −84% and −27% to −85%, respectively), followed by P. abies (−22% to −73% and −25% to −73%), P. cembra (−53% to −71% and −55% to −71%) and P. mugo (−19% to −52% and −21% to −57%). In the case of S. alba, only P. cembra did not significantly change GIs and CVGs at 2 µL/mL. P. abies reduced them by 47-74% and 27-79%, P. mugo by 25-68% and 25-71%, A. alba by 14-61% and 13-47%. On the other hand, L. multiflorum and S. alba MGT values increased up to a maximum of 16% and 21%, respectively, due to A. alba and P. abies EOs. The least effective EOs were those from P. abies and P. mugo, which were effective on L. multiflorum only at the highest concentrations (in the first case, at 10 and 20 µL/mL with a 10% and 16% increase in MGT; in the second case, at 20 µL/mL with a 14% MGT increase).
As for seedling development, A. alba, despite being significantly active only at 20 µL/mL, was the EO most capable of stunting the growth of L. multiflorum by reducing shoot length by 74%. P. cembra achieved a slightly lower result (−67%), while P. abies and P. mugo showed comparable activity (−51% and −50%). Similar percentages were obtained for S. alba, but with different efficacy of the EOs (P. cembra, −67% > P. abies, −56% > P. mugo, −50% > A. alba, −48%). Table 5 shows the final effects (after 48 h) of EOs on L. multiflorum and S. alba seedlings at 10-100 µL/mL. Their lowest tested concentrations (2 and 5 µL/mL) achieved no effect. However, all EOs significantly damaged both receiver species starting from the 10 µL/mL concentration, affecting up to 28.9% of treated seedlings with a degree of toxicity of the considered scale (Table 6) equal to 1-2 (very slight to more severe, but not lasting symptoms).   At 20 µL/mL, the phytotoxicity of EOs towards L. multiflorum varied between 2 and 4 (medium and lasting symptoms), with damage to 42.5-55.6% seedlings, 33.6-41.1% leaves, and 24.8-32.7% of their leaf surfaces. At 50 µL/mL, A. alba and P. mugo were successful in causing moderately heavy to heavy damage to L. multiflorum with symptoms affecting more than 50% of the leaf area (68.8% and 58.8%, respectively), destroying it almost completely at 100 µL/mL (≥96% damaged leaves). P. abies produced moderate to very severe symptoms (100% damaged seedlings, ≥96.7% damaged leaves, ≥54.7% damaged leaf surface) while P. cembra provoked moderate and lasting to heavy symptoms (≥86.7% damaged seedlings, ≥76.7% damaged leaves, ≥59.1% damaged leaf surface).

Effectiveness of EOs in Post-Emergence Conditions
S. alba was found to be even more susceptible than L. multiflorum to the action of EOs. At 50 and 100 µL/mL, A. alba destroyed its seedlings (100%), starting to hit them heavily at 20 µL/mL (69.4% damaged seedlings, 60.5% damaged leaves, 52.2% damaged leaf surface). At the highest concentration, P. mugo caused similar symptoms affecting 100% of seedlings and their leaves with more than 80% of the surface damaged. At 20 and 50 µL/mL, it caused medium and lasting to very heavy symptoms injuring 57.7-75.5% of seedlings. P. abies and P. cembra showed similar effects (51.1-100% and 55.6-100% damaged seedlings, 44.4-99.0% and 54.4-97% damaged leaves, 27.7-67.2% and 30.5-70.6% damaged leaf surface) with moderate and longer lasting to very severe symptoms.
Lastly, 24 h after the first treatment, only A. alba and P. cembra EOs were also significantly active at the concentration of 10 µL/mL against S. alba (data not shown).

Discussion
The chemical profile of four EOs from Pinaceae-A. alba, P. abies, P. cembra and P. mugotested for phytotoxic interactions with weeds was investigated by SPME-GC/MS. From a qualitative point of view, the EOs vapor phase was less rich in components than the respective liquid phase while, quantitatively, the percentage values of the more volatile compounds, i.e., the lower boiling temperature, prevailed. Nevertheless, in all samples, both phases were characterized by the prevalence of monoterpene compounds in accordance with the composition data of Pinaceae EOs [10][11][12][13][14][15].
A wide variety of activities, including allelopathic activity, are attributed to monoterpenes [16]. In particular, they have a marked synergistic phytotoxic action when used in combination, supporting a greater effect of crude EOs than single components [17][18][19], some of which, however, have shown a remarkable ability to suppress weed germination and growth. For example, Kordali et al. [20] demonstrated that most of the 30 investigated monoterpenes possessed significant inhibitory activity with species-specific effects and that the oxygenated ones were more active than the hydrocarbon ones. Among the most effective compounds and with comparable phytotoxicity were limonene, myrcene, α-pinene and β-pinene, the four main constituents of the EOs investigated herein.
The weed-suppressing potential of limonene was later also assessed by Vaid and co-authors [21], who discovered that even at low concentrations, it was able to inhibit germination and seedling growth in terms of height, dry weight and root elongation of Amaranthus viridis L. Zhao et al. [22] studied the impact of limonene on Chlorella vulgaris cell growth reporting that it could play allelopathic roles in cyanobacterial VOCs by reducing the photosynthetic capabilities of algae. Moreover, Jalaei and collaborators [23] identified limonene as the second most abundant compound in the EO of Dracocephalum kotschyi Boiss., capable of reducing seed germination and seedling growth of two important weeds such as Amaranthus retroflexus L. and Chenopodium album L. The phytotoxic activity of limonene and β-myrcene was further investigated, against both monocotyledonous and dicotyledonous weeds. The obtained data confirmed their potential as allelochemicals [17]. Pal Singh et al. [18] found β-myrcene as the most toxic constituent of Artemisia scoparia Waldst. & Kit. leaf EO, followed by p-cymene and limonene, suggesting that further exploration is warranted in terms of its phytotoxicity against weeds. On the other hand, Raha [24] showed that the germination of Echinochloa crus-galli L. was not significantly affected by exposure to limonene and β-myrcene, which, however, consistently and proportionally inhibited its root development to the used concentrations.
β-Myrcene was also identified as one of the major volatile constituents of Artemisia frigida Eichw. released from fresh leaves crushed and tested for their allelopathic activity with positive feedback [25]. Recently, Chen et al. [26] reported a strong phytotoxic activity of α-pinene against Elymus nutans Griseb., an important forage and ecological restoration plant species. This monoterpene was able to severely stress the target seedlings at very low concentrations. Different effects of pinene isomers against Zea mays L. were also documented by Areco et al. [27], observing a more incisive action of β-pinene compared to α-pinene. The same information was previously recorded by Nishida et al. [28]. They also verified that these monoterpenes may cause inhibition of cell proliferation in the root apical meristem of the test plants. Finally, high allelopathic activity was recorded for the EOs of Prangos pabularia Lindl. leaves, whose main compound was α-pinene [29].
As a whole, the results of this work confirmed the efficacy of Pinaceae EOs as well as their chemical content. For instance, the strong activity of EO from Pinus halepensis Mill. needles were attributed to the high content in monoterpenes, especially myrcene, but also α-pinene and β-pinene, by Aidi Wannes et al. [30] corroborating the hypothesis of Hamrouni and co-authors [31]. Likewise, the inhibiory effects of Pinus taeda L. EO on radicle elongation of Lolium species was referred to its major compounds such as α-pinene, β-pinene and limonene [32]. In contrast, for other EOs from Pinaceae leaves, including that of A. alba, our data refuted their inability to negatively affect the weed development [33], at least against S. alba and L. multiflorum.

Plant Material
Organic EOs from needle-like leaves of A. alba, P. abies, P. cembra and P. mugo (lots n. 112009, 100410, 111509 and 103006, respectively) were provided by the Bergila family business located in Falzes (Bolzano, Italy) and stored at 4 • C until use.
The target seeds of L. multiflorum and S. alba were respectively supplied by the organic farm "Terre di Lomellina" (Pavia, Italy) and by the company "Padana Sementi" (Padova, Italy).

Solid-Phase Microextraction (SPME)
To describe the chemical volatile profile of the EOs, a SPME device from Supelco (Bellefonte, PA, USA) was used for the sampling. The EOs (~0.5 mL) were individually placed into a 7 mL glass vial with PTFE-coated silicone septum. The chosen fiber was coated with 50/30 µm DVB/CAR/PDMS (divinylbenzene/carboxen/polydimethylsiloxane). Before sampling, the samples were equilibrated for 20 min at 50 • C. Subsequently, the fiber was exposed to the equilibrated headspace for 10 min to capture the components released from the EOs. Later, the fiber was inserted in the GC injector maintained at 250 • C for the desorption of collected components. Before each sampling, the fiber was regenerated at 270 • C for 20 min in the injector port.

Gas Chromatography/Mass Spectrometry (GC/MS)
A Clarus 500 model (Perkin Elmer, Waltham, MA, USA) gas chromatograph coupled with a mass spectrometer and equipped with a FID (flame detector ionization) was used to carry out the analyses. A Varian Factor Four VF-1 capillary column was used to obtain the separation of the components and helium was used as carrier gas at a constant flow of 1 mL/min. The adopted chromatographic conditions followed those noted in [34]. The mass spectra were obtained in the electron impact mode (EI) at 70 eV in scan mode in the range 35-400 m/z. The identification of the compounds was performed by matching their mass spectra with those stored in the Wiley 2.2 mass spectrum library database and by comparison of the calculated linear retention indices (LRIs), obtained using a mixture of C 8 -C 25 n-alkanes analyzed under the same conditions, with those available in the literature. Relative amounts of the compounds (percentage values) were calculated in relation to the total area of the chromatogram by normalizing the peak area. No internal standard nor factor correction were used. All analyses were carried out in triplicate.

Pre-Emergence Test with EOs in Vapor Phase
Fifteen sterilized seeds (1% NaClO) of L. multiflorum or S. alba were sown in 25 g of soil (Vigorplant ® SER CA 98 V7, Vigorplant Italia Srl, Lodi, Italy) inside Petri dishes (90 mm Ø) and wetted with 15 mL of sterilized water after placing at the same depth an appropriate number of sterile disks (6 mm Ø) impregnated with different doses of EOs (2, 20, 50 or 100 µL). In the controls, the EOs were replaced with distilled water. Samples were prepared in a sterile hood with vertical laminar flow. Then, the Petri dishes initialed and sealed with a double layer of Parafilm ® were incubated for 16 h light and 8 h darkness at 23 and 18 • C, respectively, in a climatic chamber for one week. The experimental design included four doses of each EO or distilled water × two target species × three replicates × two runs.

Pre-Emergence Test with EOs in Liquid Phase
Fifteen sterilized seeds (1% NaClO) of L. multiflorum or S. alba were sown in 25 g of soil (Vigorplant ® SER CA 98 V7) inside Petri dishes (9 cm Ø) and wetted with 15 mL of the solution prepared with different concentrations of each EO (2, 5, 10 and 20 µL/mL) and Tween ® 20 (0.2%) as surfactant. In the controls, the EOs were replaced by 0.2% Tween ® 20 solution. Samples were prepared in a sterile hood with vertical laminar flow. Then, the Petri dishes initialed and sealed with a double layer of Parafilm ® were incubated for 16 h of light and 8 h of darkness at 23 and 18 • C, respectively, in a climatic chamber for one week. The experimental design included 4 concentrations of each EO or distilled water × 2 target species × 3 replicates × 2 runs.

Post-Emergence Test with EOs in Liquid Phase
Ninety seeds total of L. multiflorum or S. alba, after germination in Petri dishes (9 cm Ø) in a growth chamber at 25 • C/16 h of light and 18 • C/8 h of dark, were transferred into 6 pots (13 cm Ø) filled with soil (Vigorplant ® SER CA 98 V7). Three weeks later, the plants with fully expanded first leaves were exposed to the treatments by spraying them every 24 h twice with the solution prepared using different concentrations (2 to 100 µL/mL) of each EO plus Tween ® 20 (0.2%) as surfactant. Control plants were instead sprayed with 0.2% Tween ® 20 solution. In all cases, the leaves were covered with small droplets, up to just before the point of outflow. The experimental design included six concentrations of each EO or distilled water × two target species × six replicates × two runs.

Phytotoxicity Indices
The effects of the four EOs on germination and development of the target species in pre-emergence conditions were described using some indices such as (i) Germination percentage (G) = (Germinated seed number)/(Seed total number) × 100; (ii) Germination Index (GI) = (7 × N1) + (6 × N2) + . . . + (1 × N15), where N1, N2 ... N15 is the number of germinated seeds (up to a maximum of 15) on the first, second and subsequent days until 7th day; the multipliers (e.g., 7, 6 . . . 1) are weights given to the days of the germination [35]; (iii) Coefficient of Velocity of Germination (CVG) = (N1 + N2 + ... + Ni)/100 × (N1T1 + ... + NiTi), where N is the number of seeds germinated every day; T is the number of days from seeding corresponding to N [36]; (iv) Mean Germination Time (MGT) = (∑D × Germinated seed number)/(∑Germinated seed number), where D is the number of days from the beginning of germination, plus the number of seeds germinated on day D [37]. The data used for their calculation were obtained after sowing, monitoring germination every day for one week and measuring the length of seedling roots and shoots on the seventh day.
The effects of the EOs on the target species in post-emergence conditions were evaluated using the scale reported in Table 6 according to Nalini and Parthasarathi [38].

Statistical Analysis
Data were assessed in IBM SPSS software by analysis of variance calculated separately for each EOs (i.e., Abies alba, Picea abies, Pinus cembra and Pinus mugo). All considered parameters (i.e., G%, GI, CVG, MGT, SVI, shoot length) for the two receiver species (i.e., L. multiflorum and S. alba) under different treatments were taken into account as dependent variables.
The one-way ANOVA and the Tukey's-b post-hoc test were used to determine the significant action (at p ≤ 0.05) of the EO treatments (i.e., the different EO doses or concentrations) on the receiver species and interpret the homogenous subsets.

Conclusions
The use of synthetic herbicides causes adverse effects on the environment and natural resources and human health. The need to identify new weed management approaches for incorporation into the increasingly sustainable development of agricultural practices has encouraged researchers to discover new potential active substances or mixtures. Based on the achieved results with A. alba, P. abies, P. cembra and P. mugo, it can be concluded that Pinaceae EOs are certainly an important source of active metabolites, in particular monoterpenes, whose potential can be exploited in the design of suitable bioherbicides, both of pre-and post-emergence, after carefully studying the involved molecular mechanism and evaluating aspects such as production costs, applicability and safety.